1. Field of the Invention
The present invention is related in general to a method and an apparatus for measuring heading changes of a road vehicle in a vehicular onboard navigation system, and in particular to a method and apparatus for measuring relative heading changes in such a system which comprises differential wheel distance measurements, velocity measurements, the wheel track and the wheel base of the vehicle. An output from a flux gate compass or other independent means for measuring changes in the heading of the vehicle may be used from time to time to compensate for errors in the wheel distance measurements.
2. Description of Prior Art
In a prior known vehicular dead reckoning onboard navigation system installed in a wheeled land vehicle, a display was provided in the vehicle for displaying a map of the roads in the vicinity in which the vehicle was driven. The vehicle was represented on the display by a symbol located in the center of the display.
In operation, as the vehicle was driven in a straight line along a road, the map was moved in a straight line on the display relative to the symbol. A representation of the road was located thereon in a position beneath the symbol. When the vehicle reached an intersection and was turned to be driven along a second road, i.e. changed heading, the map was rotated on the display relative to the symbol by a corresponding amount. The movement of the map on the display therefore corresponded, or should have corresponded, precisely to the movement of the vehicle along the roads. In practice, however, the signals used for moving the map on the display were found to suffer from a certain degree of inaccuracy and were manifested to an observer of the display by an error in the displayed position or heading of the vehicle symbol relative to the map.
One of the sources of the error in the displayed position or heading of the vehicle was found to be due to inaccuracies associated with the measuring of vehicular heading changes.
Heretofore, absolute magnetic as well as relative heading changes of a vehicle have been measured using various types of magnetic compasses, such as, for example, a magnetic flux gate compass and various types of wheel distance measuring systems and as, for example, a differential odometer system.
In the operation of the magnetic flux gate compass, as well as other types of magnetic compasses, a signal proportional to the strength of the earth's magnetic field relative to a fixed axis in the magnetic compass is generated. As a vehicle in which the magnetic compass is mounted is turned, and the angle which the axis makes with the earth's magnetic field changes, the signal generated by the magnetic compass is generated with a corresponding change in its magnitude and/or phase.
The accuracy of the output of the magnetic compass and any change therein depends on the uniformity of the earth's magnetic field in the vicinity in which the vehicle is operated. If the vehicle passes through an anomaly in the earth's magnetic field, such as may be caused by a large building, or if the compass is tilted away from the horizontal plane as when the vehicle is on a hill, banked curve, or the like, the output of the magnetic compass may indicate a heading change which did not actually occur. Such an occurrence can result in serious errors in displayed vehicle heading and position information.
In prior known simple differential odometer systems of the type used in prior known vehicular navigation systems as described above and elsewhere as, for example, U.S. Pat. No. 3,845,289, issued to Robert L. French, a pair of sensors were used for measuring the distance traveled by the front pair or rear pair of wheels of a vehicle.
In operation, the sensors in the system measured the distance traveled by one wheel in the vehicle relative to the other during a turn. From the difference in the distances measured during the turn, a computer generated a signal .DELTA..theta..sub.w corresponding to the resultant change in the relative heading of the vehicle using the equation EQU .DELTA..theta..sub.w =(D.sub.L -D.sub.R)/T (1)
where
.DELTA..theta..sub.w =change in heading PA1 D.sub.L =distance traveled by left wheel PA1 D.sub.R =distance traveled by right wheel PA1 T=distance between the two wheels, e.g. wheel track PA1 .DELTA..theta..sub.w =change in heading due to differential wheel distance measurements PA1 .DELTA.D=the difference in the distance traveled by the right and left wheels PA1 T=the wheel track PA1 V=the vehicle velocity PA1 a=a constant PA1 T.sub.F =the track not during a turn PA1 P=the ratio of single Pitman arm length to one-half the total track (approx. 1/8) PA1 .DELTA.D=the difference in distance traveled by the wheels PA1 AD=the average distance traveled by the wheels PA1 B=the distance between the front and rear wheel axles (wheel base)
In practice, the signal generated using equation (1) was often found to be inaccurate.